In the discussion of the background that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicants expressly reserve the right to demonstrate that such structures and/or methods do not qualify as prior art.
Polyamines are small positively charged molecules present in all cells. Common polyamines include putrescine, spermidine and spermine. Polyamines function in a myriad of biochemical processes including polynucleotide stabilization, transcription and translation regulation, enzyme activity modulation, iron channel regulation, and oxidative stress responses (Pegg et al., 1982, The American Journal of Physiology 243(5):C212-221; Wang et al., Polyamine cell signaling: physiology, pharmacology, and cancer research, Humana Press, Totowa, N.J., 2006). Polyamines are essential for cell growth; rates of synthesis and content both increase with increased cell proliferation. As a consequence, manipulation of polyamine metabolism has been an anti-cancer strategy, with pool depletion in tumor cells used as a surrogate marker of efficacy (Kramer and Gerner, 2004). For instance, inhibitors of the rate limiting enzymes have been evaluated clinically as anticancer and chemopreventive agents (Gerner et al., 2004, Nat. Rev. Cancer 4(10):781-792).
Because polyamines affect so many cell processes, control of intracellular pools is critical and levels are maintained within a relatively narrow range by shifts in anabolism/catabolism as well as import/export (Alhonen-Hongisto et al., 1980; Porter and Bergeron, 1988). Translation control mechanisms are known to be involved in eukaryotic cell regulation of polyamine metabolism (Coffino, 2001, Nature Reviews Molecular Cell Biology 2(3):188-194; Pegg, 2006, J Biol Chem. 281(21):14529-14532; Raney et al., 2002, J Biol Chem. 277(8):5988-5994).
Ornithine decarboxylase (ODC) is the rate-limiting anabolic enzyme. ODC is regulated by a translational control mechanism that responds to polyamine levels and involves a strong secondary structure in the mRNA 5′ UTR, fast protein turnover, and control of homodimerization required for enzymatic activity (Pegg, 2006, J Biol Chem. 281(21):14529-14532). ODC “antizyme” controls ODC by blocking homodimerization and increasing turnover; antizyme itself is regulated by a translational control mechanism involving polyamine-induced ribosomal frame-shifting (Coffino, 2001, Nature Reviews Molecular Cell Biology 2(3):188-194).
Spermidine/spermine acetyltransferase (SSAT) is the principal catabolic regulator. SSAT uses acetyl-CoA to acetylate spermidine and spermine. The acetylation reduces the positive charge of spermidine and spermine, thus making them inert and facilitating their excretion. SSAT has an extremely low basal expression and turns over faster than ODC, yet is quickly inducible to high activity when polyamines are in excess (Casero et al., 2009, Biochemical Journal 421:323-338). Experimental manipulations that increase SSAT transcription produce only limited increases in translation and, conversely, increases in translation by stimulation with polyamines can occur with little change in transcription (Fogel-Petrovic et al., 1996, FEBS letters 391(1-2):89-94; Parry et al., 1995, The Biochemical Journal 305 (Pt 2):451-458). The postulation has been that SSAT mRNA is maintained in a translationally repressed status such that cells are able to respond quickly to changes in polyamine levels by releasing the translational repression. There is some evidence to support this, but there is little information regarding the underlying molecular mechanism. Data indicate that the 3′ and 5′ UTRs are not involved, the 5′ terminus of the coding region is involved, and a repressor protein is likely to be involved, though to date, none has been identified (Butcher et al., 2007, J Biol Chem. 282:28530-28539; Parry et al., 1995, The Biochemical Journal 305 (Pt 2):451-458). Data also suggest that increased polyamine flux consequent to SSAT induction can restrict tumor growth, for instance, in prostate adenocarcinoma (Kee et al., 2004, J Biol Chem 279(38):40076-83, Epub 2004 Jul. 13; Babbar et al., 2011. Recent Results Cancer Res. 2011; 188:49-64; Simoneau et al., 2008, Cancer Epidemiol Biomarkers Prev. 2008 February; 17(2):292-9).
During recovery and preservation organs are anoxic, as they are in ischemia, and following transplantation they are reperfused. Reperfusion may result in ischemia-reperfusion injury (IRI). IRI may also arise following resumption of blood flow to an organ when blood flow is interrupted, such as following brain injury or myocardial infarction. IRI is estimated to be responsible for 10% of early graft loss in the case of transplanted livers (Amersi et al., J. Clin. Invest. 1999; 104:1631).
Agents that repress SSAT translation may possess therapeutic activity in disorders or diseases characterized at least in part by increased SSAT activity. Exemplary diseases having such increased SSAT activity include ischemia-reperfusion injury, stroke and myocardial infarction. In particular, prevention of SSAT translation provides an opportunity for prevention of IRI, including IRI after liver or renal transplant, myocardial infarction (Han et al. (2009) Int J Cardiol 132:142-144; Zahedi et al. (2009) Am J Physiol Gastrointest Liver Physiol 296:G899-G90) and brain injury (Zahedi et al. (2010) J Neurotrauma 27:515-525).
In each of these conditions, ischemic injury has been suggested to increase SSAT activity to a level that causes cell toxicity. Studies with SSAT knockout animals have confirmed that renal ischemia-reperfusion injury is at least partially SSAT dependent because SSAT knockout animals have very mild injury compared to wild type animals (Zahedi et al. 2009).
Ischemic reperfusion injuries such as acute renal failure, acute liver failure, stroke, and myocardial infarction are prevalent causes of morbidity and mortality. In particular, kidney ischemic reperfusion injury is the leading cause of acute renal failure and dysfunction of transplanted kidneys. Treatment options for IRI are few.
There is also an unmet need in the art to identify the mechanism underlying translational repression in eukaryotic cell regulation of polyamine metabolism and to identify agents that relieve translational repression. There is also a need to identify agents that increase or decrease translation of SSAT for use in therapies where increased or decreased expression of SSAT is desired. The present disclosure addresses this need.